Worm melterand extruder

ABSTRACT

THE SPECIFICATION DISCLOSES AN IMPROVED WORM MELTER OR EXTRUDER FOR HIGHLY VISCOUS PLASTIC MELTS WHICH NORMALLY FLOW IN LAMINAR FASHION. THE DISCHARGE END OF SUCH MELTED OR EXTRUDER IS USUALLY FORMED BY AN END SURFACE OF A WORM AND AN INSIDE SURFACE OR A MOUTHPIECE. THE PRESENT INVENTION PROVIDES MEANS PROJECTING FROM SAID END SURFACE AND SAID INSIDE SURFACE TO BREAK UP THE MELT PASSING THROUGH THE DISCHARGE END TO EFFECT A TURBULENE AND MIXING WITHIN THE MELT TO PRODUCE A MORE UNIFORM PRODUCT.

Dec. 26, w SCHWALM 27,539

WORM MELTER AND EXTRUDER Original Filed Oct. 17. 1967 WILHELM SCHWALMUnited States Patent Ofi Re. 27,539 Reissuecl Dec. 26, 1972 ice Int.(:1. B2sb 27/02 us. 01. 259-491 4 Claims Matter enclosed in heavybrackets II] appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE The specification discloses an improved wormmelter r extruder for highly viscous plastic melts which normally flowin laminar fashion. The discharge end of such melter or extruder isusually formed by an end surface of a worm and an inside surface of amouthpiece. The present invention provides means projecting from saidend surface and said inside surface to break up the melt passing throughthe discharge end to effect a turbulence and mixing within the melt toproduce a more uniform product.

BACKGROUND OF THE INVENTION Field of the invention-Description of theprior art The invention relates to a homogenization device for wormmelters or extruders which contain a worm rotatable in a casing, the endsurface of said worm being arranged at a short distance from the casingmouthpiece provided with a discharge passage.

In the processing of plastics, such as thermoplastic materials, in wormmelters or extruders non-homogeneities or non-uniformity occur veryeasily in the product discharged. Such non-homogeneities may arise, forexample through temperature diiferences between the casing wall and theworm surface as well as through insufiicient mixing of the material.This phenomenon is detectable especially at throughput rates whichresult in reduced residence times for the plastic materials.

Such non-homogeneities have a very adverse effect on the productquality, for example, deterioration of the physical properties of theproducts produced from the melts and/or great differences in theseproperties. These non-homogeneities in the products are especiallynoticeable if the discharge material is fed over connecting passages orpipe lines to several different processing stations, such as tools,spinning devices, nozzles or the like.

Since the high viscosities of such plastic melts almost always result inlaminar flow pattern with no turbulence or thorough mixing in the pipelines, even if gear wheel pumps are interposed, the plastic melts maarrive at the next processing station in flows of varying temperatureand thereby with differing viscosity. Thus, for example, it was observedthat in the spinning of two separate threads from a spinning nozzle, asa result of the nonhomogeneous melt, the amount of melt appearing on theone side of the spinning nozzle was always greater than that appearingon the other side, whereby there resulted great deviations in the denierof the threads. Furthermore, in supplying several parallel processingstations with a non-homogeneous melt, it was found that the melt to eachstation varies constantly in viscosity, which results, inter alia, inconsiderable differences in the physical values of the threads spun atthe individual stations.

Attempts to eliminate such non-homogeneities in highly viscous melts byheat treatment during the How through pipelines, even over relativelylong periods of time, have led to no success whatsoever. Such failure isprobably due to the constantly laminar flow conditions, especially sincethe high viscosities of the melts require certain minimum pipe crosssections to keep the pressure drops within reasonable limits.

In order to arrive at a good homogeneous end product together with highproduction rates, various proposals have been made for worm extrusionpresses. In these proposals, essentially quality reductions on increasesof worm speeds are compensated by increasing the pressure inthe plasticcomposition. Such a pressure build-up can be achieved, for example, in aworm with conical end surface which lies opposite a corresponding coneof the mouthpiece, by an axial-wise shifting of the worm, which changesthe size of the emergence gap or the outlet and thereby changes thepressure on the plastic composition within the melter or extruder (seeDer -Plastverarbeiter,

No. 1, 1962). In another proposal (German patent specification1,l45,787), the plasticized material is subjected to a high compressionin the extrusion press, depending on its properties on one or morestages of the path to be covered in the press cylinder, by anarrangement in the extrusion press compression sections consisting of anannular collar interrupting the worm courses, whose outside diameter isequal to or somewhat smaller than the diameter of the bore of the presscylinder and a flat annular groove provided in the bore of the presscylinder with differing depth in the axial direction. By axial-wiseshifting of the worm, the effect of the compression sections may bemodified.

The above described proposals are rather expensive and they are notsuccessful for processing all types of plastics in every kind ofextruders or worm melters.

In contrast to the above, the present invention provides ahomogenization device for worm melters and extruders which is extremelysimple and even permits mounting on nearly all forms of extruders'orworm melters subsequent to their original installation.

OBJECTS It is an object of the present invention to provide a wormmelter or extruder for viscous plastic melts which has an increasedmixing effect on said melts and which produces products of increaseduniformity.

It is another object to provide an improved worm melter or extruderinexpensively. It is a further object of the invention to provide aninexpensive method for converting existing worm melters and extruders toincrease their mixing effect and product uniformity.

Still other objects can be gathered from the following disclosure.

THE INVENTION The above objects are achieved, according to theinvention, by means projecting from the end surface of the worm as wellas on the interior surface of the mouthpiece opposing said end surface,said projecting means lie on circles coaxial and central to the wormaxis. Preferably, the projecting means are arranged in such a way thatthe circles of projections on the end surface of the worm and thecircles of projections on the inner surface of the mouthpiece alternatesuccessively, with a small radial spacing thereinbetween.

Through the device according to the invention, there is achieved asurprisingly effective mixing and homogenization action, because theprojecting means turn against one another in comb fashion toautomatically break up the melt running through thereby creatingturbulence and mixing in the melt. This makes the use of other expensivehomogenizing devices normally unnecessary although such otherhomogenizing devices may be used in addition to the device of thepresent invention without any difficulty. Moreover, the device of theinvention can be economically and conveniently installed on existingworm melters and extruders to increase their efficiency.

In an experiment, there were spun from a two-part spinning system whichwas fed from a worm melter two equal cord base threads of nylon 6. Witha throughput of 366 g./min., the thread of the first spinning stationhad a tearing strength of 7.5 g./denier and a breaking stretch of 18.0%,while the thread from the second spinning station had a tearing strengthof 6.8 g./denier and a breaking stretch of 20.8%. Thereafter, thehomogenization device according to the invention was installed in theworm melter and the experiment repeated under otherwise entirelyidentical conditions. The thread from the first spinning station now hasa tearing strength of7.2 g./den. and a breaking stretch of 20.2% whereasthe thread from the second spinning station now has a tearing strengthof 7.07 g./den. and a breaking stretch of 19.8%. By simple installationof the homogenization device according to the invention, the differencesin the properties of the threads from the two spinning stations wereconsiderably reduced-namely, for the tearing strength the difference wasreduced from 0.7 g./den. to 0.13 g./den. and for the breaking stretchthe difference was from ca. 3.0% to 0.4%. These reduced differences nowlie within the normal range of allowable tolerances. It is possible toreduce these differences still further, when practicing the presentinvention, by a temperature modification at the worm melter: atemperature alteration on a worm melter not equipped from the deviceaccording to the invention is without effect.

Further details of the invention are explained below with the aid of theappended drawing.

In the drawing, which represents a longitudinal sectional view of thedischarge end of a worm melter, there is shown a worm 2 in a casing 1,the end of which worm terminates in a conical surface 3. On the casing 1there is placed a mouthpiece 4 which has a cylindrical output aperture5. Aperture 5 widens conically in the direction of worm 2 in such amanner that the mouthpiece 4 has a conical surface 6 lying parallelopposite the worm 2.

On the conical surface 3 of the worm 2 there are arranged circularlyabout the worm axis two rows of projections 7 situated at a regulardistance from one another. On the conical surface 6 of the mouthpiece 4there are situated one or more corresponding circles of projections 8(only one circle of projection 8 is shown in the drawing), which lie onboth sides of the circle of projections 7 and have a small radialspacing from this. The projections have studlike form and consist, inthe example represented, advantageously of the heads of screws set inthe worm 2 and the mouthpiece 4.

As shown in the drawing the projections 7 and 8 are flat a d wide beingsubstantially greater in width dimension than in height dimension. Thedistances between surfaces 3 and 6 defining the narrow annular dischargepassage is approximately equal or slightly less than twice the heightdimension of the projections.

When the melt, which normally flows in laminar form by reason of itshigh viscosity, enters the gap between the conical surface 3 of worm 2.and the conical surface 6 of the mouthpiece 4 it is torn up and brokenin its flow by the projections 8 and 7 which move past one another incomb form to produce turbulence and mixing. Through this processexisting non-homogeneities in the melt are eliminated.

It is obvious that on the worm end and on the inside surface of themouthpiece there can be arranged more or less than three circles ofprojections and that the projections there may have the most diverseforms, and may, for example, have a round, oval or irregular crosssection. The device can be made especially simple and effective,however, by putting in screws since-aside from the advantages inmanufacturing techniquetheir angular (polygonal) heads are extremelyadvantageous for the mixing process.

Likewise, the worm end and the inside of the mouthpiece do not have tobe shaped conically, but may have any other form. If, for example, theworm is cut at a right angle, and/or its end is at a relatively largedistance from the discharge end of the mouthpiece, then the projectionscan extend, as viewed from the worm axis, in radial direction on theworm end and the corresponding cylindrical part of the mouthpiece wallor casing wall. There are many different possibilities for the mountingof the pro-.

jections. It is only important that the projections on the worm and themouthpiece wall interact in comb fashion in such a way that the melt istorn up in its flow and set in turbulence.

What is claimed is:

[1. In a worm melter or extruder for viscous plastic melts of the typecomprising a rotatable worm disposed within a housing, the region ofcommunication between an end surface of said worm and a correspondingend surface of said housing defining a discharge passage, theimprovement comprising providing a plurality of means projecting fromsaid end surface of said worm and from said end surface of said housing,said means on each of said end surface form circles concentric with saidworms axis, and the circles on said end surface of said worm and thecircles on said end surface of said housing alternate and intermesh incomb fashion when said worm is rotated, thereby interrupting the flow ofsaid melts through said discharge passage to create turbulence andmixing within said melts] [2. A melter or extruder according to claim 1wherein said means are screws set into said surfaces] [3. A melter orextruder according to claim 2 wherein said screws have angular heads][4. A melter or extruder according to claim 1 wherein said surfaces areconically shaped to define a conically annular discharge passage, saidend surface of said worm having 2 circles of said means thereon and saidend surface of said housing having one circle of said means, said circleon said housing being positioned between the circles on said worm] 5. Ina worm melter 0r extruder for viscous thermoplastic melts characterizedby laminar flow through a conical discharge passage comprising arotatable worm disposed within a housing, the region of communicationbetween an end surface of said worm and a corresponding end surface ofsaid housing defining a narrow annular discharge passage, theimprovement comprising providing a plurality of closely-spaced flatprojections projecting from said end surface of the worm and said endsurface of the housing into said passage, which projections cooperate tocreate turbulent flow of plastic melt in said passage when the worm isrotated; said projections being substantially greater in width than inheight dimension and positioned on each of said end surfaces in the formof a circle concentric with the axis of said worm, said projections onsaid worm alternating and intermeshing in comb like fashion with theprojections on said housing when said worm is rotated, therebyinterrupting the flow of said melt through said discharge passage tocreate turbulence and mixing within said melt.

6. The melter of claim 5 wherein said projections are screw heads set onsaid surfaces.

7. The melter of claim 6 wherein said heads are polygonal in shape.

The following references, cited by the Examiner, are of record in thepatented file of this patent or the original patent.

UNITED STATES PATENTS 3,000,618 9/ 1961 Oakes 1812 C 3,169,752 2/1965 DeLaubal'ede l812 SEX 2,810,159 10/1957 Teichmann 18-12 SE 6 Anderson.Greene 18-12 Munger et a1. Cummings et a1.

FOREIGN PATENTS Germany.

H. A. KILBY, JR., Primary Examiner US. Cl. X.R.

